This invention relates to electric storage batteries and more particularly to batteries wherein the electrochemically active cell elements (i.e., interleaved positive and negative polarity plates) have some plates enveloped in a sheath of battery separator material.
Battery separators are well known in the art and comprise microporous membranes interjacent adjacent electrodes in a cell element and serve to separate and electrically isolate one (e.g., positive electrode) from the other (e.g., negative counter-electrode). Such separators have sufficiently small pores to suppress interelectrode dendrite growth but sufficiently high porosity to permit electrolyte mobility within the cell. A typical such material comprises sintered PVC particles such as described in Bahler et al U.S. Pat. No. 3,551,210.
Some battery manufacturers envelope all the positive or negative plates in individual sheaths or envelopes formed from the separator material. Enveloping the plates in separator sheaths improves handling of the elements and reduces the possibility of dendrite-produced, interelectrode shorting at the edges of adjacent positive and negative plates (i.e., edge shorting). A typical such sheath comprises an elongated strip of separator material folded transverse its length and heat sealed together along two edges. The fourth edge is typically left open.
In the battery, the sheath rests on the bottom of the container and supports its enveloped plate above the bottom of the container by an amount equal to the thickness of the sheath at its bottom edge. One end of the enveloped plate projects out the open edge of the sheath (usually by means of a plate lug) and is electrically joined to other electrode plates of like polarity within the cell element. Such joining may be effected by means of a conventional plate strap extending the length of the element or by simply bundling and fusing the several plate lugs together into a post such as disclosed in United States patent application Ser. No. 259,975 filed May 4, 1981 in the names of William J. Chafin et al and assigned to the assignee of the present invention. In a single cell battery, the plate straps or posts are appropriately connected through the container wall to the output terminals of the battery. In a multicell battery, the cell element of one cell is electrically coupled to similar elements in the next adjacent cell compartments by means of intercell connectors joined to the plate straps or posts. In either case, the ends of the plates which are joined together are effectively anchored against movement to either the battery container external wall or intercell partition separating adjacent cell compartments. The distal edge of the plate (i.e., opposite the anchored lug-bearing edge) is neither joined to other plates nor anchored to the container and hence is free to move up and down within the container incident to vibration, rough handling, etc.
Such movement stresses the plate lug causing fatigue and potential breaking of the lug of a plate unrestrained against such movement. In the case of sheathed plates, the sheath resting on the bottom of the container tends to support the encased plate against such potentially destructive movement. Unsheathed plates, on the other hand, run a high risk of lug breakage when the plates are located above the bottom of the container so as to be aligned with the sheathed plates. This problem is particularly acute in batteries where the plate lugs project from the sides of the plates (i.e., rather than the tops) and the stress on the lug is at a maximum. Moreover it is difficult during assembly to properly axially align unsheathed plates with sheathed plates due to the different outside dimensions of each. In this regard, the sheathed plates are supported above the bottom of the container by a distance equal to the thickness/width of the edges of sheaths, while the unsheathed plates tend to settle to the bottom of the container out of axial alignment with the sheathed plates. This settling cants the plate and stresses the lugs thereof.
It is an object of the present invention to provide a storage battery having plates of one polarity sheathed in battery separator material and interleafed with unsheathed plates of opposite polarity wherein the sheaths support the unsheathed plates above the bottom of the battery container and in substantial alignment with the sheathed plates.